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[SCG53-P18] **Estimation of pore fluid overpressures for tensile cracking at depth of shallow slow earthquakes**

Keywords:Slow earthquake, Stress, Crack, Fluid, Subduction

Pore fluid pressure

*P*_{f}is important for understanding slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélanges, eastern Kyushu, SW Japan. In the coastal region of the Makimine mélanges (Late Cretaceous Shimanto accretionary complex of SW Japan; temperature = 300–350°C, Palazzin et al., 2016), the mélange preserves quartz-filled shear, foliation-parallel veins and tension vein arrays. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal σ_{1}-axis trending NW–SE and a sub-vertical σ_{3}-axis, and the driving pore fluid pressure ratio*P** (*P** = (*P*_{f}– σ_{3}) / (σ_{1}– σ_{3})) is ~0.1. When the pore fluid pressure exceeds σ_{3}, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds σ_{3}is the pore fluid overpressure Δ*P*_{f}(Δ*P*_{f}=*P*_{f}*– σ*_{3}). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999).*P*_{f}in the case of the Makimine mélanges are ~280 MPa (assuming depth = 10 km, density = 2750 kg/m^{3}, tensile strength = 5 MPa and Young’s modulus = 7.5–15 GPa). The normalized pore pressure ratio*λ** (*λ** = (*P*_{f}–*P*_{h}) / (*P*_{l}–*P*_{h}),*P*_{l}: lithostatic pressure;*P*_{h}: hydrostatic pressure) is ~1.03 (*P*_{f}>*P*_{l}).